1. Field of the Invention
The object of the present invention is a method and a device for the slant-range correction a short-range radar.
2. Description of the Prior Art
As is known, a radar transmits cycles of pulse signals which are reflected by surrounding obstacles and return to the radar transmitter. On reception, the radar receives a signal called a video signal, consisting of a succession of signals of the same duration, called recurrences, each recurrence consisting of all the echos received in response to a transmitted pulse signal. After processing, the video signal is displayed on a screen. The range (D) of a moving object with respect to the radar center is proportionate to the time (t) taken by this pulse to return: ##EQU1## with c, the speed of the electromagnetic waves. In general, the video signal is sampled and digitalized. The distance D is then given by: ##EQU2## where E is the number of samples from the beginning of the recurrence or, again, the sequence number of the sample if these samples are numbered sequentially from the beginning of the recurrence, and Te is the sampling period.
Certain radars are used to monitor air traffic at an airport (especially the traffic of aircraft on the ground) or again, traffic in a harbour, i.e. they are used to monitor traffic in zones close to the radar center. In this type of application, the radar must be at a raised level, so that its monitoring function is not hampered by land obstacles: it is placed then on a tower, the height of which may be about a hundred meters. The range measurements made by the radar are obviously done with respect to the radar center, whereas only the ground projections of these distances are useful: the ranges measured must therefore be corrected to express them with reference to the foot of the tower. If this is not done, there is a resulting error in the positioning of the echos which is equal, at the maximum, to the height of the tower. Furthermore, the radar echos are displayed on a screen which generally has a map of the ground infrastructures (landing strips, taxiways, etc.) superimposed on it. The above positioning error then results in a lack of coincidence between the radar video and the map, which is quite detrimental to the efficient monitoring of the zone considered.
Various methods can be used to cope with this lack of coincidence.
One of these methods of the prior art entails the drawing up of a special map of the installations on the ground in relation to the radar center by using, for example, a reflector on a vehicle which travels through the various strips of the airport. This is a cumbersome method to use: the maps must be specially made and conventional cartographic surveys cannot be used. Furthermore, this method can be used only to superimpose the map so that it precisely matches the radar video, and any additional information displayed on the screen, for example an indication of scale, must also be corrected.
Another method is to make the correction with respect to the very notion of the distance associated with a given sample: in this method, a sample is no longer taken to represent a constant elementary increment in distance but is associated with a distance datum which must be calculated according to the sequence number of this sample (hence, to the relative position of the obstacle with reference to the radar center). This method is fairly cumbersome to use and has the disadvantage of being unusable in existing installations unless they are physically altered.